Nits
Nits are the eggs of head lice, which appear as a yellow or white speck attached to the hair shaft. According to Dr. Lois Dusdieker at the University of Iowa Children's Hospital, head lice lay their eggs close to the scalp because of the body heat. If you find nits that are further from the scalp than a few millimeters, it is a good indication that they have already hatched.
After hatching, the empty nit egg will remain attached to the hair shaft until physically removed with a special comb. Nits will typically hatch in 7 to 9 days; they can survive off the human head for up to 10 days but cannot hatch below 68 degrees Fahrenheit.
Nymphs
According to the New Jersey Department of Health and Senior Services, the nymph stage of head lice appears as a miniature of the adult. Taking its first feeding of blood soon after hatching, the nymph will feed every three to six hours for the next nine days as it passes through two more nymph stages to reach adult status.
Adults
The Illinois Department of Public Health reports that adult head louse can live up to a month on a human host. During this time, the female will typically deposit 90 eggs on the hair shafts. Adult head lice measure in length between 1/16 to 1/18 inch and cannot survive more than 48 hours without blood; without a host the adult louse can live 2 to 4 days at 74 degrees Fahrenheit.
